The long term objectives are to elucidate the role of mannose-binding protein (MBP) in protecting non-immune hosts from overwhelming infections or parasitic infestations. The overall objective is to investigate the newly described capacity of MBP to activate the alternative complement pathway (ACP) and to enhance serum bactericidal activity. Potential interactions between MBP, C-reactive protein (CRP), and serum amyloid P (SAP) in control of complement activation will be studied. Using enteric gram negative bacteria (primarily Salmonella species) bearing various amounts of mannose in their LPS, characteristics of susceptible organisms, of MBP, and of complement which allow participation in this immune mechanism will be examined. similar studies will be performed using Leishmania major, a parasite with a distinct, mannose-rich lipophosphoglycan during its infective stage. The possibility that MBP may interact with C-reactive protein (CRP) or with serum amyloid P (SAP) to modulate complement activation will be explored. To achieve these goals, the following specific aims will be pursued: I. Determine structural requirements for MBP necessary to enhance C3 deposition on susceptible bacteria by studying the capacity of MBP proteolytic fragments or hybrids to enhance C3 binding to bacteria. II. Investigate the mechanism for enhanced ACP activation by MBP by examining its interaction with each component of the ACP -C3, Factors B&D, and properdin - and the regulatory proteins - Factors H&I - using LPS- bearing liposomes and bacteria III. Determine LPS components which interact with MBP by comparing its interaction with rough mutants and smooth wild type strains, by attempting to block MBP activity with mannose or N-acetylglucosamine or with MAb directed against each LPS chemotype, and by examining LPS lengths optimum for interactions with MBP and complement IV. Determine the mechanism for serum bactericidal activity mediated by MBP by investigating the effects MBP has on lytic terminal complement components and on the acceptor sites for C3 amide bond formation V. Determine if MBP enhances complement deposition on L. major VI. Determine if complement activation by MBP is affected by CRP or SAP These seem extremely important concepts to understand about MBP, a circulating protein which prevents invasion of cells by HIV, and also is involved in immune response to an organism, Salmonella, which is particularly difficult to eradicate in AIDS patients.